This application is based on patent application No. 9-228386 filed in Japan, the contents of which is hereby incorporated by reference.
This invention relates to an optical image projector of the ray splitting type or light mixing type for projecting an optical image, which is used, for example, in a projector for displaying a color video image and adopts a dichroic mirror.
As one kind of optical image projector, there have been marketed liquid crystal projectors. A liquid crystal projector acts as a means for displaying a large image and enlargedly projects an optical image from liquid crystal panels which is obtained by modulating the luminance of illumination light in accordance with a video signal on a screen via a projection lens.
The construction of such a liquid crystal projector is described with reference to FIG. 8. In FIG. 8, light irradiated from an illumination optical system 51 is split into rays in three wavelength ranges of R (red), G (green) and B (blue) by dichroic mirrors 52, 53.
Specifically, a ray in the wavelength range of R reflected by the dichroic mirror 52 illuminates a liquid crystal panel 56 for R after being reflected by a full-reflection mirror 54 and transmitting through a field lens 55. Rays in the wavelength ranges of G and B transmit through the dichroic mirror 52. Thereafter, the ray in the wavelength range of G illuminates a liquid crystal panel 58 for G after being reflected by the dichroic mirror 53 and transmitting through a field lens 57. Further, the ray in the wavelength range of B illuminates a liquid crystal panel 64 after transmitting through the dichroic mirror 53, being introduced to a relay optical system including two lenses 59, 60 and two full-reflection mirrors 61, 62 and then transmitting through a field lens 63.
Further, optical images of the respective colors formed by the three liquid crystal panels 56, 58, 64 are combined into one image by a dichroic prism 65.
Specifically, the optical image of R formed by the liquid crystal panel 56 is incident on the dichroic prism 65, propagates straight therein and is reflected at right angles by a first dichroic coating surface 65a after being incident thereon at 45xc2x0 to emerge toward the projection lens 66. Further, the optical image of G formed by the liquid crystal panel 58 is incident on the dichroic prism 65, propagates straight therein without being reflected by the first and second dichroic coating surfaces 65a, 65b to emerge toward the projection lens 66. Furthermore, the optical image of B formed by the liquid crystal panel 64 is incident on the dichroic prism 65, propagates straight therein and is reflected at right angles by the second dichroic coating surface 65b after being incident thereon at 45xc2x0 to emerge toward the projection lens 66.
As described above, the optical images of the respective colors formed by the three liquid crystal panels 56, 58, 64 for R, G, B are caused to emerge toward the projection lens 66 after being combined by the dichroic prism 65 with an optical axis and the directions of the optical images aligned. The combined optical image is enlargedly projected on the screen via the projection lens 66.
In the case that an optical integrator including first and second lens arrays is used as the illumination optical system 51, the first lens array splits light from a light source and incident thereon into a plurality of rays by a plurality of lenses thereof, and a plurality of rays from the first lens array are projected on the display surfaces of the respective liquid crystal panels 56, 58, 64 in a superposed manner by the second lens array.
However, in the above conventional liquid crystal projector, the rays incident on the first and second dichroic coating surfaces 65a, 65b are not necessarily completely parallel. This results in color nonuniformity.
In FIG. 9, the construction of FIG. 8 is simplified in order to simplify the following description. In other words, a typical arrangement of the first and second dichroic mirrors 52, 53 is provided between the field lens 57 and a second lens array 72 for introducing the rays to the liquid crystal panel 58 for G, so that the rays from the second lens array 72 can transmit through both the first and second dichroic mirrors 52, 53.
As shown in FIG. 9, the liquid crystal panel 58 for G is telecentrically illuminated via the field lens 57 by setting an angle distribution of the rays from a plurality of secondary light source images 73 formed in the vicinity of the first lens array 72 by a plurality of lenses of the first lens array 71 within a specified range. Assuming that L denotes a distance between the second lens array 72 and the field lens 57, a focal length f1 of the field lens 57 is set at L in order to ensure the telecentric illumination. A most intensive ray al which is from point a in the position of an aperture of the first lens array 71 and is at the center of an intensity distribution of light energy which contributes most to the projected image is incident on the dichroic prism 65 as converged light although it should be perpendicularly incident on the display surface of the liquid crystal panel 58, i.e., should be incident on the dichroic coating surfaces 65a, 65b of the dichroic prism 65 at 45xc2x0 In other words, at points A, B located at opposing ends of the liquid crystal panel 58, the most intensive ray al contributing most to the projected image and located at the center of the intensity distribution of light energy is incident on the dichroic coating surfaces 65a, 65b at 45xc2x0 xc2x1xcex1. Thus, wavelengths to be cut off by the dichroic coating surfaces 65a, 65b are shifted due to an incident angle dependency of the cutoff values of the dichroic coating surfaces 65a, 65b. As a result, the color is differed in positions of the projected image corresponding to points A and B, causing a color nonuniformity in the projected image.
The cutoff values of the dichroic coating surfaces 65a, 65b have incident angle dependencies. Thus, if the cutoff value for an incident angle of 45xc2x0 is set at 580 nm for the entire dichroic coating surface 65a, the cutoff value is shifted as much as the incident angle is shifted. Then, as shown in FIG. 10, points of inflection of the spectral distribution of the projected light shift between the opposite ends (left and right ends) of the screen, which causes a color nonuniformity. As a result, the color becomes nonuniform in the projection screen.
Japanese Unexamined Patent Publication No. 4-142530 discloses a projection type liquid crystal display device using a coating of a varying thickness. In this device, a wavelength selecting characteristic in each position is made equal by changing the wavelength selecting characteristic of a dielectric multi-layered coating of a dichroic prism for the image combination according to an inclination of a main ray to a projection lens, thereby avoiding the creation of color nonuniformity in a projected image. Such a method using the coating of a varying thickness also has the problem of a difficult maintenance of the coating characteristics.
Also, Japanese Unexamined Patent Publication No. 4-223456 discloses a projection type liquid crystal display device additionally including a trimming filter. In this device, there are provided a dichroic mirror for selectively transmitting or reflecting a light at a specified wavelength, a first optical device constructed by a dichroic prism for the image combination, and a trimming filter as a second optical device for cutting off light components in a wavelength range corresponding to a change in the incident angle of a light on the first optical device such that a wavelength range of the transmitted light or reflected light falls within a specified region even if the incident angle of the light on the first optical device varies. Accordingly, a projected image free from color nonuniformity and having high color purity can be reproduced on a screen in color separation and color combination. However, such a method with an additional trimming filter requires an additional coating surface, increasing the number of parts, and more maintenance for the coatings. Thus, the coating maintenance becomes more difficult and a production cost increases.
It is an object of the present invention to provide an optical image projector which has overcome the problems residing in the prior art.
According to an aspect of the present invention, an optical image projector comprises: an illuminator which emits a number of light rays; a separator which separates each light ray from the illuminator into a plurality of color component rays having different wavelengths from one another; an optical image generator which generates a plurality of optical images utilizing separated color component rays; an image combiner which combines the plurality of optical images into a single optical image; and an optical system which is provided between the illuminator and the image combiner such that the most intensive light rays strike the image combiner at a predetermined incident angle.
According to another aspect of the present invention, an optical image projector comprises: a light source which irradiates light; an optical integrator which converges the light from the light source into a number of secondary light sources; a separator which separates each light ray from the secondary light sources into a plurality of color component rays having different wavelengths from one another; an optical image generator which generates a plurality of optical images utilizing separated color component rays; an image combiner which combines the plurality of optical images into a single optical image; and an optical system which is provided between the optical integrator and the image combiner such that the most intensive light rays strike the image combiner at a predetermined incident angle, the optical system including a lens whose focal length f1 satisfies the condition defined by the following equation:
{(D+d)/D}xc2x7Lxe2x89xa6f1xe2x89xa6{D/(Dxe2x88x92d)}xc2x7L
wherein d denotes an interval between secondary light sources formed in the vicinity of a center optical axis of the optical integrator, D denotes a dimension of an area to be illuminated, L denotes a distance between the optical integrator and the lens.